Hydrodynamics of topological defects in nematic liquid crystals

We show that back-flow, the coupling between the order parameter and the velocity fields, has a significant effect on the motion of defects in nematic liquid crystals. In particular the defect speed can depend strongly on the topological strength in two dimensions and on the sense of rotation of the director about the core in three dimensions.Comment: 4 pages including two figure

Colorado Distance Horse Bowl Tournament

Horse bowl is one of the most popular activities among Colorado 4-H horse project members. To participate in the traditional program, youth must travel to the contest\u27s host county. Our goal was to create an online horse bowl program to allow youth to participate wherever access to the Internet was available. The Web-based program WebCT was used to implement the Colorado DHBT. Youth participated as individuals or on a team in three separate contests, constituting one tournament. After the tournament, participants were asked to complete an online survey to evaluate the effectiveness of the program

Interfacial motion in flexo- and order-electric switching between nematic filled states

We consider a nematic liquid crystal, in coexistence with its isotropic phase, in contact with a substrate patterned with rectangular grooves. In such a system, the nematic phase may fill the grooves without the occurrence of complete wetting. There may exist multiple (meta)stable filled states, each characterised by the type of distortion (bend or splay) in each corner of the groove and by the shape of the nematic-isotropic interface, and additionally the plateaux that separate the grooves may be either dry or wet with a thin layer of nematic. Using numerical simulations, we analyse the dynamical response of the system to an externally- applied electric field, with the aim of identifying switching transitions between these filled states. We find that order-electric coupling between the fluid and the field provides a means of switching between states where the plateaux between grooves are dry and states where they are wet by a nematic layer, without affecting the configuration of the nematic within the groove. We find that flexoelectric coupling may change the nematic texture in the groove, provided that the flexoelectric coupling differentiates between the types of distortion at the corners of the substrate. We identify intermediate stages of the transitions, and the role played by the motion of the nematic-isotropic interface. We determine quantitatively the field magnitudes and orientations required to effect each type of transition.Comment: 14 pages, 12 fig

Renewal of comparator stimuli

ABSTRACT Prior research has found a) recovery from overshadowing as a result of posttraining extinction of comparator stimuli (i.e., the overshadowing stimulus), and b) context modulation of conditioned responding to Pavlovian stimuli (i.e., renewal). The present research brought together these two findings by investigating whether comparator stimuli are subject to contextual control. In a Pavlovian conditioned suppression situation, rats were exposed to an overshadowing procedure (i.e., AX1) in one context and then received extinction of the overshadowing cue (i.e., A2) in the same or a different context. If AX1 training and subsequent extinction of A occurred in the same context, animals exhibited recovery of responding to the target cue (i.e., X) regardless of the test context. However, if AX1 training and extinction of A occurred in different contexts, behavior depended on the test context. If X was tested in the overshadowing context, overshadowing was observed. But if X was tested in the context where A had been extinguished or in a third (neutral) context, overshadowing was not observed. Thus, context modulates comparator effects in a manner somewhat similar to how it modulates simple Pavlovian responding. The notable exception was that robust responding to both A and X was observed in the neutral context, which is problematic for most contemporary theories of learning